Isotope labelled 1-Cyclopropyl-8-ethoxy-6,7-difluoro-1,4-dihydro-4-oxo-3-quinolinecarboxylic Acid Ethyl Ester is used in the preparation of alkoxyquinolone antibacterial agents. Also an impurity in the synthesis of Moxifloxacin (M745000), a fluorinated quinolone antibacterial.